Accurate location estimates become more important as an increasing number of mobile devices populate our environment. For example, autonomous devices, such as autonomous vacuum cleaners, warehouse robots, drones and the like use their current location estimate to perform their tasks efficiently and without causing accidents.
One widely used solution for determining an accurate location estimate is the Global Positioning System (GPS). GPS uses satellites and determines the current location using propagation times from known satellite positions. However, the satellite signals are absorbed by building structures and therefore, the accuracy of GPS is limited, if at all possible, indoors and in narrow streets with high buildings where most satellites are obscured.
As an alternative to using satellite signals, mobile devices can use radio frequency radiation from stationary devices, such as fixed location Wi-Fi hotspots. The mobile devices can determine the propagation time from the fixed location and calculate a location estimate.
FIG. 1 illustrates a prior art system 100 comprising a mobile device 101, a first stationary device 102 and a second stationary device 103. Mobile device 101 receives radio frequency (RF) radiation from first stationary device 102 and determines a propagation time of the radiation from the stationary device 102. Using the speed of light of 300,000 km/s as the propagation velocity, the mobile device 101 determines a first distance 104 from the first stationary device 102. As a result, the location estimate of mobile device 101 is somewhere on a first circle 105 with its radius being the calculated distance 104 from the first stationary device 102. The centre of the first circle 105 is the location of first stationary device 102, which may be retrieved from a lookup table using the frequency or signature data of the RF radiation.
Similarly, mobile device 101 receives RF radiation from second stationary device 103 and determines a second distance 106 from second stationary device 103. The estimated location is somewhere on a second circle 107 around second stationary device 103 with its radius being the calculated second distance 106 from the second stationary device 103. The mobile device can then calculate the location estimate as the intersection 108 of the first circle 105 and the second circle 107. While system 100 provides a good accuracy of the estimated location 108, the calculations rely on direct line of sight propagation of the RF radiation.
FIG. 2 illustrates an indoor scenario 200 comprising again a mobile device 201, a first stationary device 202 and a second stationary device 203. Mobile device 201 calculates the distance 204 from the first stationary device 202 to define a first circle 205. In contrast to FIG. 1, the indoor scenario 200 comprises first obstacle 206 and second obstacle 207. First obstacle 206 absorbs RF radiation such that the signal received by mobile device 201 from second stationary device 203 directly, that is, through obstacle 206, is attenuated to a level near or below the level of noise. As a result, mobile device 201 does not detect the received radiation as a useful signal. Instead, second obstacle 207 reflects the RF radiation and the reflected radiation reaches mobile device 201. This is referred to as “non-line-of-sight” (NLOS) propagation. Mobile device 201 detects the RF radiation and determines that the RF radiation originated from the second stationary device 203, such as by querying a lookup table for the detected frequency or data signature.
It is practically impossible for mobile device 201 to find out whether the received RF radiation has been reflected or received directly. Therefore, mobile device 201 calculates a distance 208 to second mobile device 203 which is the length of the propagation path and longer than the direct distance. As a result, intersecting the first circle 205 with a second circle 209 having the NLOS distance 208 as its radius results in a location estimate 209 that is inaccurate.
FIG. 3 illustrates another scenario 300 where the mobile device moves along a straight travel path as indicated by a solid line 301. The mobile device detects RF radiation from stationary device 302 and calculates the current location estimate as described above. An obstacle 303 blocks the line-of-sight RF radiation propagation 304 and 305 for a section of the travel path 301 but allows the RF radiation to propagate around its edges as indicated at 306 and 307. The solid dots, such as example dot 308, indicate the location estimates. As the path 301 continues under the obstacle 303 from left to right, the location estimate becomes increasingly inaccurate. In this case, the inaccuracy basically follows a cosine function and returns back to good accuracy when the mobile device receives radiation in the line of light 305.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.